Undesirable yawing movement correcting means for gas-cushion vehicles

ABSTRACT

AUTOMATICALLY TO CANCEL OUT YAWING MOMENTS GENERATED BY RELATIVE WINDS ON A GAS-CUSHION VEHICLE, A VERTICAL FIN IS MOUNTED THEREON FOR ROTATION BY WIND FORCES ABOUT AN AXIS DISPLACED FROM THE VEHICLE&#39;&#39;S CENTER OF GRAVITY. THE INCIDENCE OF THE FIN IS ADJUSTED AS IT ROTATES TO GENERATE THE PREDETERMINED YAWING MOMENTS REQUIRED FOR THIS PURPOSE, BY A TAB, THE SETTING OF WHICH IN RELATION TO THE FIN IS GOVERNED BY A CAM. TO PRODUCE YAWING MOMENTS SUFFICIENT TO RENDER THE VEHICLE NEUTRALLY STABLE WHEN OPERATING IN ALL RELATIVE WIND DIRECTIONS TWO FINS ARE PROVIDED.

United States Patent lnventor John Walter Flower Mangotsfield, Bristol, England Appl. No. 806,995 Filed Mar. 13, 1969 Patented June 28, 1971 Assignee National Research Development Corporation London, England Priority Mar. 18, 1968 Great Britain 12716/68 UNDES IRABLE YAWING MOVEMENT CORRECTING MEANS FOR GAS-CUSHION VEHICLES Primary Examiner-A. Harry Levy Allomey--Cameron, Kerkam & Sutton ABSTRACT: Automatically to cancel out yawing moments generated by relative winds on a gas-cushion vehicle, a vertical fin is mounted thereon for rotation by wind forces about an Chins 23 Drawing Figs axis displaced from the vehicles center of gravity. The in- U.S.Cl 180/117, cidence of the tin is adjusted as it rotates to generate the 244/82 predetermined yawing moments required for this purpose, by

Int. Cl 860v l/00, a tab, the setting of which in relation to the fin is governed by a B60v1/14 cam. To produce ya'wing moments sufficient to render the Field of Search 180/117, vehicle neutrally stable when operating in all relative wind 120; 244/82, 87, 91 directions two fins are provided.

fga -K/q PAT EN TED Junzelsn 3,587; 770

' SHEET 3 BF 8 PATENTED JUN2 8191i SHEET l 0F 8 PATENTEDJUN28I9YI 35817-70 sum s are M FIGQ.

PATENTEU JUN28 1971 587-, 7,7 0

sum 6 0F 8 Initial tab position F Final cab posit/01') centre on 671 axis/ CG. Q

F/G.77b.

RW Position ofcam at height 0/ cam follower afltar vertical movement I FIG. 77c.

UNDESIRABLE YAWING MOVEMENT CORRECTING MEANS FOR GAS-CUSHION VEHICLES This invention relates to gas-cushion vehicles, that is to say to vehicles for travelling over a surface and which in operation are supported above that surface, at least in part, by a cushion of pressurized gas for example air, formed and contained beneath the vehicle body.

With conventional aircraft, which are designed so that when in flight the relative wind is always from ahead, varying in fact only by 10 or so degrees from this direction, directional sta bility can be achieved quite easily by the addition of a fixed vertical fin towards the rear, with a portion of the fin movable to provide for directional control.

Gas-cushion vehicles, however, are required to operate in relative winds coming from any direction, and with such vehicles which have little or no contact with the surface over which they travel, and hence are particularly susceptible to horizontally acting aerodynamic forces, directional stability and control presents a problem.

A vehicle will normally have a shape such that when a relative wind comes from one direction, the vehicle will be inherently stable or have positive stability, that is to say the vehicle will tend to maintain its heading in relation to the wind,

e.g. after the manner of a weathercock, and when the wind comes from another direction the vehicle will be inherently unstable or have negative stability, that is to say the heading of the vehicle relative to the wind will not be maintained.

What is occuring in these circumstances is that the horizontally acting aerodynamic forces, the magnitude and direction of which are of course determined by the magnitude and direction of the relative wind, which will be the vector sum of the existing ground speed and direction of the vehicle and wind, will in general exert a yawing moment on the vehicle.

Considering a vehicle in equilibrium, in conditions when the vehicle is positively stable and a change in the relative wind direction will result in a change of yawing moment, which will tend to return the vehicle to its original heading relative to the wind. ln conditions when the vehicle is negatively stable a change in the relative wind direction will result in a change of yawing moment which will tend to increase the change in the heading of the vehicle relative to the wind.

In certain circumstances, for example when cruising at speed, it may be useful that a vehicle should have positive sta bility. A corollary of positive stability is, however, that in maintaining its heading relative to the wind, when the wind direction changes the heading of the vehicle relative to the surface over which it is travelling will be correspondingly changed. When, as is often the case with air-cushion vehicles, it is required to maneuver them in relatively confined spaces, it may be preferable to have less positive stability or neutral stability in order to improve the response of the vehicle to steering controls.

It is accordingly an object ofthis invention to provide means whereby yawing moments generated by horizontally acting aerodynamic forces on a gas-cushion vehicle can automatically be cancelled out, so that a vehicle fitted with the means in accordance with the invention may be rendered neutrally stable, that is to say it will be unaffected by relative winds coming from any direction, since the net yawing moment generated by the wind will be zero.

According to the invention a gas-cushion vehicle is provided with at least one vertically disposed fin, mounted on the vehicle for rotation under the action of wind forces about a substantially vertical axis, ahead of the aerodynamic center of the fin, and displaced from the center of gravity of the vehicle, and means automatically to adjust the incidence of the fin in relation to the relative wind direction, so that in operation in the aerodynamic forces produced by the fin generate yawing moments on the vehicle in opposition to yawing moments generated by the wind on the remainder of the vehicle.

With such an arrangement it is then possible to render a vehicle substantially neutrally stable for at least a majority of relative wind directions.

Because in certain wind directions, depending upon the position of a fin in relation to the center of gravity to the vehicle, forces produced thereby may have no or an insufficient moment arm about the center of gravity to generate the required yawing moment, to enable a vehicle to be rendered substantially neutrally stable when operating in a relative wind from any direction, it will be necessary to provide at least a second fin, the axis of rotation of which will be appropriately spaced from the axis of rotation of the other fin such that when the forces produced by one fin may not act through a sufficient moment arm, the forces produced by the other fin may act through a moment arm sufficient to generate the required yawing moment.

The means for automatically adjusting the incidence of the fin may include any known form of aerodynamic control means, in turn controlled by an appropriate scheduling control in dependence upon relative wind directions which will be sensed.

Preferably the aerodynamic control means comprise an aerodynamic control surface movably attached to the fin. In a preferred embodiment of the invention the aerodynamic control surface is in the form of a tab attached to the trailing edge of the fin for angular movement with respect thereto, and a cam is provided with which a follower connected to the tab is arranged to cooperate, the cam being provided with a predetermined profile such that as the tin rotates into the direction of a relative wind, the tab is set by the cam in a predetermined angular position in relation to the tin, so that the aerodynamic forces produced by the fin and tab generate the required yawing moment.

If desired means may be provided to adjust the incidence of the fin, other than the said automatic means, to provide a facility to occasion the fin to generate an elected yawing moment, for example for steering purposes. Alternatively, or in addition, means may be provided to adjust the operation of the aerodynamic control means so that the fin may be automatically adjusted to provide a vehicle with a degree of positive stability.

The invention is described below with reference to the accompanying drawings in which:

FIG. 1 is a side elevation of a typical gas-cushion vehicle to which the present invention may be applied;

FIG. 2 is a diagram indicating how yawing moments generated on a vehicle by relative winds, may vary with wind direction, due to inherent aerodynamic characteristics of a gas-cushion vehicle;

FIG. 3a is a vector representation of forces which can e generated by a pivotally mounted vertical fin for varying incidence;

FIG. 3b is similar to FIG. 3a, indicating the manner in which yawing moments that may be produced by the fin, when mounted on a vehicle, vary for different positions of the fin axis in relation to the center of gravity of the vehicle;

FIGS. 4a, b and c are diagrams of a gas-cushion vehicle fitted with a single pivotally mounted fin and tab assembly in accordance with the invention, indicating yawing moments which may be generated thereby to render the vehicle neutrally stable;

FIG. 5 is a diagram of a gas-cushion vehicle fitted with two pivotally mounted fin and tab assemblies in accordance with the invention, to ensure that for all wind conditions the required yawing moments may be generated thereby;

FIGS. 6 and 7 are elevation andplan views respectively of a fin and tab assembly, in accordance with the invention, with the tab controlled by a cam;

FIGS. 8a, b and c are diagrams similar to FIG. 5, indicating how the positioning of the fin and tab assemblies may depend upon the basic aerodynamic characteristic of a vehicle;

FIGS. 9 and 10 are elevation and plan views respectively of a modification of the fin and tab assembly of FIGS. 6 and 7;

FIGS. Ila, b and c are explanatory diagrams of how the as sembly of FIGS. 9 and 10 may be operated to provide a vehicle with a degree of positive stability, in addition to rendering it neutrally stable;

FIGS. 12a, b and c are explanatory diagrams indicating how a cam controlling the tab of a fin and tab assembly, may be operated to enable the assembly to be used for steering purposes; and

FIGS. 13 and 14 are elevation and plan views respectively of a mechanism for adjusting a tab-controlling cam.

With reference to FIG. 1 a typical air-cushion vehicle 1, of the plenum chamber-type, is illustrated, adapted to travel over a water surface 1 supported by a cushion 3 of pressurized air formed beneath the vehicle body 4 and contained thereunder by flexible wall structure or skirt 5, which is attached to and depends from the periphery of the vehicle body. A skirt of the form illustrated is the subject of Pat. No. 3,420,330.

The vehicle supporting air-cushion 3 is formed by drawing in ambient air through an intake 6 compressing it in a compressor 7 driven by an engine 8 nd discharging it beneath the vehicle body through a duct 9. The vehicle may be propelled by any convenient means, such as an air screw propulsion unit 10. Directional control is conveniently provided by a fin l and rudder 16.

It is to be understood that the present invention is applicable to any form of gas-cushion vehicle and is in no way limited to gas-cushion vehicles of the particular form exemplified above. While it is believed that the present invention will be of greatest use with a gas-cushion vehicle which has no means, such as keels, to contact the surface over which it is designed to operate, it may nevertheless be of use with vehicles, having surface contacting means.

A gas-cushion vehicle will usually be symmetrical about its fore and aft axis, and with the single exception of a vehicle which is axi-symmetric about a vertical axis through its center of gravity, a given vehicle will, in general, generate a particular yawing moment when the wind is from a particular direction. The value of this yawing moment will be proportional to wind speed squared (V A typical manner in which the yawing moment may vary with wind direction is indicated in FIG. 2. The curve indicated therein which results from plotting the yawing moment over V against the wind direction, will be unique for a given vehicle. As indicated, when the wind angle is from ahead or astern the yawing moment will be zero because of vehicle symmetry. At other angles the yawing value will, in general, be nonzero and will form a smooth curve without discontinuities, probably of an S" form as shown.

If a vehicle has a basic aerodynamic characteristic as indicated by the curve, then it will be stable or have a positive stability in relative wind conditions ranging from dead ahead to approximately 90 to port or starboard, that is to say when the vehicle is operating in the range of conditions indicated by the region A of the curve. On the other hand when operating with the relative wind coming from between dead astern to approximately 90to port or starboard the vehicle will be unstable or have negative stability. Operation of a control fin or rudder with which the vehicle may be provided will change the yawing moment, and indeed to obtain trim, i.e. to maintain a chosen heading of the vehicle relative to the surface over which it is travelling, a rudder will normally be moved to a position which results in the net yawing moment generated by the relative wind or on the vehicle, being zero, but the slope of the curve will be virtually unaltered and the regions of the positive and negative stability will remain substantially the same, as is indicated by the dotted line' in FIG. 2 which represents the vehicle trimmed for position F. For the purposes of this explanation the following sign convention has been adopted:

Clockwise rotation, angles and moments are termed positive and conversely anticlockwise rotation, angles and moments are termed negative.

Vehicle forces and directions are only considered in relation to the wind and zero vehicle angle is when the apparent. or relative, wind is from ahead.

Considering a vehicle in equilibrium with the relative wind coming from the starboard bow, as at position F, upon positive rotation of the vehicle heading relative to the wind, e.g. to

point F, a negative yawing moment, indicated by the line F --G, will be generated tending to return the vehicle to its original heading at F. Thus negative slope of the curve indicates positive stability, and conversely positive slope of the curve indicates negative stability.

Consider now the effect of a pivotally mounted vertical fin. FIG. 3a is a vector representation of forces generated by such a fin for varying incidence. It will be noted that the direction of the forces which can be generated by the fin are always at an angle greater than to the wind direction indicated by arrow RW. This is because the resulting force from the fin includes drag components. Considering now the fin mounted on a vehicle, the forces which may be produced by the fin will generate some moment about the center of gravity of the vehicle, which moments may be positive or negative depending upon whether the incidence of the fin is positive or negative in relation to the wind and the position of the fin in relation to the center of gravity of the vehicle. The direction of the moments will vary as indicated in FIG. 3b, Thus, if the center of gravity of the craft is at point A, a force X produced by the fin acting about moment arm X, will generate a positive (clockwise) yawing moment, whereas a force Y produced by the fin, acting about moment arm Y will produce a negative (anticlockwise) yawing moment. If the center of gravity of the vehicle in relation to the pivot axis of the fin is moved to point B by changing the heading of the vehicle in relation to the wind, force X produced by the fin acting about moment arm X" will generate a negative (anticlockwise) yawing moment and force Y acting about moment arm Y will generate a positive (clockwise) yawing moment. On the other hand if the center of gravity moves to point C yawing moments produced by forces such as X and Y, which will act about moment arms X' and Y' respectively, will be positive (clockwise) in both cases.

Accordingly while it will be seen from the above that in principle, depending upon the size of a fin and the displacement of its axis from the center of gravity of a vehicle, a yawing moment of any desired value may be generated, if an adjustable fin is to be provided whereby it can be arranged to generate yawing moments equal in magnitude to those generated by the basic craft characteristic but opposite in sign, so that the total moment may be reduced to zero, a single fine will not suffice to make this possible for all relative wind directions. This problem may simply be overcome by providing two or more fins arranged so that at least one can generate an effective yawing moment for any particular wind direction.

This, however, leaves the problem of how yawing moments produced by the basic aerodynamic characteristic of a vehicle can, other than empirically be balanced or controlled, i.e. how for each relative wind direction appropriate fin incidences can be automatically determined, so as to give the vehicle neutral stability for all wind directions.

According to a preferred embodiment of the invention this is achieved by the use of at least one vertical airfoil surface or fin mounted on the vehicle body for rotation, under the action of wind forces, about a substantially vertical axis, ahead of its aerodynamic center displaced from the center of gravity of the vehicle, to the trailing edge of which the fin a control surface or tab is attached, the angular setting of which in relation to the fin is arranged automatically to be determined by a cam mechanism.

How this may be done will now be explained. With only aerodynamic forces acting on a fin that is freely pivoted about an axis ahead of its aerodynamic center, or in practice somewhere between the leading edge of the fin and the quarter chord position, the fin will tend to weathcock into the direction of the relative wind. If, however, the fin is fitted with an auxiliary control surface or tab, movement of the tab relative to the fin will generate an aerodynamic moment about the fin axis forcing the fin to rotate until some angle is reached where the moment of the fin plus the tab about the fin axis is zero. With the fin set at some incidence to the wind, the fin will have some side forces on it, and assuming the fin to be mounted on a vehicle with its pivot point displaced from the vehicle's center of gravity, the force acting on a moment arm about the center of gravity will provide a yawing moment on the vehicle. The value of the yawing moment will obviously depend upon the size of the fin and tab assembly and the displacement of its pivotal axis from the center of gravity of the vehicle. Also, the yawing moment produced will vary as the moment arm about the center of gravity changes in the manner made apparent in connection with FIG. 3b.

Further to explain this and referring to FIGS. 4a, b and c, these are diagrams of gas-cushion vehicle fitted with a single pivoted fin and tab assembly in accordance with the invention. In each FIG. the relative wind direction is indicated by the arrow RW and the fin is shown set at two different incidences (the same in each case) in relation to the wind, one in full and one in dotted lines, to indicate yawing moments which may be generated by the fin on the vehicle. In further detail FIG. 40 indicates a vehicle operating in a relative wind from ahead. With the fin and tab assembly set at the incidence indicated in full lines, a force x will be produced by the fin, which acting about a moment arm x, will produce a negative yawing moment on the vehicle. Similarly with the tab and fin assembly set at the incidence indicated in dotted lines the fin will produce a force y which, acting about moment arm y, will produce a positive yawing moment on the vehicle.

In FIG. 4b the vehicle is shown operating in a stern wind. With the fin and tab assembly set in the full line position it will produce a force x which, acting about moment arm x", will generate a positive yawing moment on the vehicle, the fin and tab assembly, when set in the dotted line position producing a force y acting about the moment arm y", will generate a negative yawing moment.

It will accordingly be seen from these diagrams that when operating in either of these conditions an adequate yawing moment can be expected to be generated by the fin and tab assemblies to combat yawing moments produced on the vehicle due to its inherent aerodynamic characteristic.

In the case of the vehicle operating in a relative beam wind as indicated in FIG. 40, however, it will be seen that with the fin and tab assembly set at either of the two incidences indicated, the forces x and y act about much reduced moment arms x' and y' and furthermore the yawing moments will, in this example, both be positive yawing moments. Both the fact that the moment arm about which the force produced by the fin and tab assembly may be very small, and the fact that it cannot be assured that the yawing moment will be of the required sign, means that a single fin and tab assembly will not be adequate to produce yawing moments to combat those due to the inherent characteristics of vehicle for all wind directions. There is obviously also the case in which the force produced by the fin may have no moment arm about the center of gravity of the vehicle.

Accordingly to ensure that adequate yawing moments of the required magnitude and sign can be produced for all wind directions, it will be necessary to provide a gas-cushion vehicle with at least two fin and tab assemblies, as for example indicated in FIG. 5.. This represents a gas-cushion vehicle operating in a similar relative wind direction as in FIG. 4c, in which condition the port fin will be substantially ineffective to produce a yawing moment on the vehicle. On the other hand the force which can be produced by the starboard fin and tab assembly, e.g. the force y, acting about moment arm y, can produce an entirely adequate yawing moment.

Since the inherent aerodynamic characteristic of a vehicle can, as indicated at the outset, be determined, which charac teristic may typically be of the nature indicated in FIG. 2, for a given fin and tab assembly mounted at a given distance along a given axis from the vehicles center of gravity, the required angular setting of the tab in relation to the fin for any given relative wind, to give the fin the necessary incidence, so as to produce the required balancing moment, can be determined. From this information the profile ofa cam to be followed by a follower connected to the tab can be determined such that the tab angles are automatically determined by the cam shape, as the fin rotates under the action of the wind, to generate the required forces or moments to balance the yawing moments produced by the basic vehicle characteristic. Changes in the way tab deflection varies with fin directions can be effected either by changing the geometry of the cam or by movement of the cam, as will be explained further below.

The arrangement may conveniently be as shown in FIGS. 6 and 7. Referring to these FIGS., a fin 15a is provided with a pivot 18, whereby the fin will be vertically mounted on a vehicle body (not shown), for rotation under wind forces, about a substantially vertical axis located at a fixed position displaced from the center of gravity of the vehicle. A tab 16a is hinged at 17 to the trailing edge of the fin. Beneath the fin, centered on the same axis as the pivot 18, is a cam 19, on the edge ofwhich a cam follower 20, connected to the tab 16a, runs so as to control the setting of the tab relative to the fin 15a. As will be seen from FIGS. 6 and 7, the cam 19 has a central opening 19 therein providing a clearance between the cam and the pivot 18 which permits adjustment of the position of the cam relative to the pivot.

The local geometry of the cam 19, as the fin is rotated under the influence of the wind, tending to maintain its leading edge to the wind, will thus generate some tab angle, moving the fin to some angle of incidence to the wind. This may tend to lead to some further small change in tab deflection and hence incidence, but this factor can readily be catered for in the same design. The result will, therefore, be that for a particular vehicle the cam geometry will determine the fin incidence and hence the yawing moment required in all wind conditions.

It will have been gathered from the foregoing that various factors must be taken in account to determine the optimum position for the fin and tab assemblies in accordance with the invention. Hence to keep fin sizes to reasonable proportions it is obviously desirable that the axes on which the fins are mounted should be displaced as far as is practicable from the center of gravity of the vehicle on which they are mounted. Also when more than one fin is provided, as explained in relation to FIG. 5, it is desirable to arrange that in conditions when one fin is ineffective another fin is at a position to provide maximum effectiveness. A further consideration is the effect that the drag components of the force produced by a fin may have on the moment arm through which the forces act.

To explain this, with reference to FIG. 8, a gas-cushion vehicle is diagrammatically shown, which is assumed to have the inherent aerodynamic characteristic of FIG. 2. In FIG. 8a the vehicle is shown operating at an angle of plus 90 in relation to the relative wind. Under these conditions from FIG. 2 it will be seen that the vehicle has an inherent negative yawing moment requiring a positive yawing moment to be applied to produce a net yawing moment of zero. If, as shown, a pair of fin and tab assemblies are mounted at the rear of the vehicle, because as mentioned in connection with FIG. 3a, due to drag components forces produced by the fins will always act at an angle greater than 90 relative to the apparent wind direction, the moment arms, about which forces x and y may act about the center of gravity of the vehicle, are reduced. Hence, in the case of a vehicle having a basic characteristic of FIG. 2, it may be desirable to mount the fin and tab assemblies at the front. If this is done, as indicated in FIG. 8b, it will be seen that the effect of the drag components can be turned to advantage to increase rather than decrease the moment arms about which the forces produced by the fins, act. FIG. indicates that the same benefits are available with the vehicle operating at a heading of minus relative to the apparent wind.

It should be understood that the vehicle characteristic indicated in FIG. 2 is only exemplary and it is entirely possible that a vehicle may have a characteristic which is the inverse of that illustrated. In this case, for the reasons given above, it would be advantageous to provide a pair of fin and tab assemblies at the rear.

It will be seen that many options are open regarding the number and positioning of fin and tab assemblies in accordance with the invention and the invention is not limited to the number or the positioning of the fins as illustrated. Generally, apart from displacing the pivot axis of each fin as far as is practical from the center of gravity of a vehicle, it is equally desirable when more than one fin is provided, for lines joining their axes to the center of gravity to be suitably angu larly disposed, preferably at an angle of the order of 90.

It will be seen that rendering a vehicle neutrally stable in the manner described above, results in the vehicle being automatically trimmed to maintain a chosen heading relative to the surface over which it is travelling. While this will be of great benefit in that the vehicle will require no active, i.e. pilotoperated, control to combat yawing moments generated by relative winds, whether these vary in direction or not, it may, in certain circumstances, be desirable, rather than give the vehicle completely neutral stability, to provide a degree of positive stability. This is because with a vehicle possessing positive stability the facility is provided of being able to trim it for a certain heading relative to the wind which the vehicle will maintain automatically.

A way in which a fin and tab assembly, arranged to provide a vehicle with neutral stability, may be modified to provide a degree of positive stability, is indicated in FIGS. 9 to 11. The assembly shown in FIGS. 9 and 10 is basically the same as that of FIGS. 6 and 7, but the tab is formed in two parts. With such an arrangement one part, say 16a, can be used to obtain neutral stability and the other parts 16b can be used to provide positive stability and trim control. The tab 16a will be controlled by a cam [90, similar to the cam 19 of FIGS. 6 and 7, and the second tab may be controlled by a second cam 19b, having a central opening 19. This second cam 19b is circular and initially has its center on the axis of the fin, so as to give no tab deflection. The cam.l9b has the form of a frustum of a cone with the larger diameter at the bottom, as shown. If the cam 19b is moved a small distance horizontally along a line generally perpendicular to that joining the center of gravity of the vehicle to the axis of the fin a, as permitted by the open ing 19" and as indicated by the arrow M in FIG. 110, this results in a movement of the tab 16!; and hence puts the vehicle out of balance, that is to say the trim of the vehicle is disturbed. Note that if the cam follower 20b is offset, as indicated in FIG. 10, from the chord line of the fin, the direction of movement of the cam should in fact be correspondingly altered to allow for this, as indicated by the arrow M.

To bring the vehicle back into trim the cam 1912 will be vertically adjusted so that the tab is returned to its zero deflection position, as indicated in FIG. llb. The result of these cam movements is such that while no change in tab angle is effected, movement of the fin will result in changes in the setting of the tab 16b. Hence, a change in the direction of the relative wind will create a tab deflection.

Consider the changes associated with positive rotation of a vehicle relative to the wind (or anticlockwise movement ofthc wind relative to the vehicle). The fin will tend to follow the relative wind or move anticlockwise relative to the vehicle body. With the relative wind coming from the direction indicated by the arrow RW in FIG. 11b, when the cam follower of tab 1612 would be cooperating with the cam 19b in the region indicated at A,where due to the adjustment thereof the cam will have an effective slope as shown, this would give an anticlockwise or negative rotation of the tab leading to positive rotation of the fin and hence a negative yawing moment about the center of gravity of the vehicle. This would be a moment tending to restore the vehicle to its original position and hence the vehicle would be positively stable.

With the relative wind coming from the opposite direction, when the cam follower of the tab 16b is cooperating with the region of the cam indicated at B, there would be clockwise or positive rotation of the tab leading to negative rotation of the fin, again producing a negative yawing moment about the center of gravity of the vehicle, which would give the vehicle positive stability. The situation would in principle be the same for all other relative wind directions with limiting cases, when the wind was at right angles to the line joining the fin axis to the center of gravity of the vehicle, i.e. when the cam follower of the tab 16b would be cooperating with the points C and D of the cam [9b, where the fin would be ineffective in providing positive stability, the cam 19b having no effective slope in these regions. Further the forces produced by the fin would have no or insufficient moment arm about the center of gravity of the vehicle. With more than one fin, however, as previously explained, positive stability could be arranged to be given by one fin whilst the other was ineffective, provided the fins were not nearly in line with the center of gravity of the vehicle.

From the above it will be seen that the amount of horizontal movement of the cam 19b will directly determine the amount of positive stability provided, and vertical movement of the cam 19b will regain equilibrium and hence give trim. While the provision of two tabs, one 16b to give neutral stability and the other 16b to provide positive stability with trim, is thought to be convenient, this may in fact not be necessary. A single tab operated by one cam could in principle be arranged to serve both functions. The shape of the cam, in plane, would be determined so as to give neutral stability and the cam would be vertically tapered with horizontal movement of its giving stability and vertical movement giving trim.

The heading of a vehicle will not always be determined completely by its reaction to changes in relative wind direction. Direct control will be required for maneuvering, and it has been assumed that this will be provided for by means, e.g. a conventional rudder, other than the fin and tab assemblies of the invention. However, these may, if desired, be arranged at least to assist in direct steering of a vehicle.

Consider a vehicle that has been rendered neutrally stable by one or more fin and tab assemblies in which the tabs were operated as previously described. In order to generate a positive yawing moment when the fin is downstream of the center of gravity of the vehicle, i.e. when operating in a relative headwind, as indicated in FIG. 12a, the fin would need a negative incidence or the tab a positive deflection. As indicated, this could be obtained by horizontal movement of the cam in a direction away from the center of gravity. Similarly, when operating in a relative wind from astern, as indicated in FIG. 12b, where the fin is upstream of the center of gravity, in order to generate a positive yawing moment, the fin would need a positive incidence or a negative tab direction, but this again would require horizontal movement of the cam away from the center ofgravity ofthe vehicle.

As before, horizontal adjustment of the cam can in principle be effective to provide elected yawing moments in all wind directions, but the effectiveness of such an adjustment will fall off when the relative wind is from the beam, as indicated in FIG. 120, for the reasons previously made clear. Again this may be catered for by providing one or more further fin assemblies.

With reference to FIGS. 13 and 14, a mechanism is shown for adjusting a tab control cam for the purposes described above in connection with FIGS. 11 and 12. Thus the mechanism includes two orthogonally arranged hydraulic rams or jacks 30 supporting a table 40 on which is mounted cam 19. The piston rod 31 of each jack 30 is connected, by means of keys 32, to the undersurface of the table, engaging keyways 32a cut into its undersurface and extending normal to the axis of the associated piston rod. The key and keyway arrangement allows relative movement between the cam 19 and the piston rods 31 in directions normal to the axes of the piston rods.

Each jack 30 is provided with hydraulic flow and return lines 33, 34, and by means of selector valve units (not shown) the jacks 30 can be used to move the cam 19 either in a fore and aft direction or from side to side.

The cam I9 is supported on the table 40 by jacks 35 provided with hydraulic flow and return lines 36 and 37 by which the cam can he raised and lowered.

While it is preferred, if for no other reason than because of the basic simplicity of the arrangement, to control the incidence of a pivoted fin by means of a tab in turn controlled by a cam as described above, it will be seen that this is not essen tial. The incidence and/or the forces produced by a fin could obviously be adjusted in a variety of ways. Thus aerodynamic control means of any known form, including for example jet flaps or other boundary layer control arrangements, could be used to control forces produced by the fin. Further, operation of these aerodynamic control means could be governed by a control mechanism of any suitable form, including appropriate means for sensing relative wind directions and an associated scheduling control device operative automatically to adjust the aerodynamic control means to occasion the fin to produce the reQuired yawing moment.

Finally, although the typical gas-cushion vehicle illustrated in FIG. 1 is shown as having an air screw propulsion unit, it will be appreciated that if such a unit is used to propel the vehicle it is undesirable to have the fin and tab assemblies of the present invention operating directly in the slipstream. it may be desirable on this account, for a vehicle fitted with fin and tab assemblies in accordance with the invention, to pro vide some other form of propulsion and if air propulsion systems are used these might preferably be jet propulsion systems arranged so that the air intakes and jet nozzles did not produce air flows such as would adversely affect the desired weathercocking action of the fin.

lclaim:

l. A gas-cushion vehicle comprising at least one vertically disposed fin, means mounting said fin on the vehicle for rotation of said fin about a substantially vertical axis under the action of wind forces on said fin, said axis being located ahead of the aerodynamic center of the fin, and displaced from the center of gravity of the vehicle, and means connected to said fin and operable automatically in response to rotation of the fin about said axis under the action of wind forces for so adjusting the incidence of the fin in relation to the relative wind direction that in operation the aerodynamic forces produced by the fin generate yawing moments on the vehicle in opposition to yawing moments generated by the wind on the reminder of the vehicle, whereby the vehicle may be rendered substantially neutrally stable for at least a majority of relative wind directions.

2. A gas-cushion vehicle as claimed in claim 1 including means additional to the said automatically operable means for further adjusting the incidence of the fin so as to generate an elected yawing moment.

3. A gas-cushion vehicle as claimed in claim 1 in which the said automatically operable means include aerodynamic control means associated with the fin.

4. A gas-cushion vehicle as claimed in claim 3 in which the aerodynamic control means comprise an aerodynamic control surface movably attached to the fin.

5. A gas-cushion vehicle as claimed in claim 4 in which the aerodynamic control surface comprises a tab connected to the trailing edge of the fin for angular movement about a substantially vertical axis with respect to the fin.

6. A gas-cushion vehicle as claimed in claim 5 including a cam centered on substantially the same axis as the axis of rotation of the fin and a cooperating follower connected to the tab, the cam being provided with a predetermined profile such that as the fin rotates into the direction ofa relative wind the tab is moved by the cam through said follower into a predetermined angular position in relation to the fin so that the aerodynamic forces produced by the fin and tab generate the required yawing moment.

7. A gas-cushion vehicle as claimed in claim 6 including means for moving the cam relative to the fin to adjust'the angular position of the tab so as to generate elected yawing moments by the fin.

8. A gas-cushion vehicle as claimed in claim 7 in which the follower is arranged to operate in a horizontal plane and the cam is movable horizontally along a line joining the axis of the fin and the center ofgravity of the vehicle.

9. A gas-cushion vehicle as claimed in claim 6 in which the cam is so profiled, and which includes means for so adjusting the cam, that the tab and hence the fin is automatically adjusted, as the fin rotates under the action of relative winds, so as to provide the vehicle with a degree of positive stability.

10. A gas-cushion vehicle as claimed in claim 9, in which the follower is arranged to operate in a horizontal plane and the cam is movable horizontally along a line generally perpendicular to a line joining the axis of the fin and the center of gravity of the vehicle, and also in a direction at right-angles to the plane of the first-mentioned movement.

11. A gas-cushion vehicle as claimed in claim 6 including a second tab connected to the trailing edge of the fin for angular movement with respect thereto, a further cam for controlling the second tab, and means for so moving the further cam that the second tab and hence the fin is automatically adjusted as the fin rotates under the action of relative winds so as to provide the vehicle with a degree of positive stability.

12. A gas-cushion vehicle as claimed in claim 11 including a second follower connected to the second tab to cooperate with the further cam, the follower of the second tab being arranged to operate in the horizontal plane, the further cam being circular in plan but tapered in vertical section, and the further cam being movable horizontally along a line generally perpendicular to a line joining the axis of the fin and the center of gravity of the vehicle, and also vertically.

13. A gas-cushion vehicle as claimed in claim 1 including means for so adjusting the operation of the said automatically operable means that the fin may be automatically adjusted to provide the vehicle with a degree of positive stability.

14. A gas-cushion vehicle comprising at least two vertically disposed fins, means mounting said fins on the vehicle for rotation of said fins about substantially vertical axes under the action of wind forces on said fins, the axes of rotation of said fins being located ahead of the aerodynamic centers of the fins, and displaced from the center of gravity of the vehicle and from each other, and means connected to said fins and operable automatically in response to rotation of the fins about their axes under the action of wind forces for so adjusting the incidence of the fins in relation to the relative wind direction that in operation the aerodynamic forces produced by the fins generate yawing, moments on the vehicle in opposition to yawing moments generated by the wind on the remainder of the vehicle, whereby the forces produced by one fin do not act through a sufficient moment arm to generate the required yawing moments, the forces produced by the other fin produce sufficient yawing movements to ensure that the vehicle may be rendered substantially neutrally stable for all relative wind directions. 

